How could simulating the ports as open be useful?
I've read an article which describes how to simulate the close ports as open in Linux by eBPF. That is, an outside port scanner, malicious actor, will get tricked to observe that some ports, or all of them, are open, whereas in reality they'll be closed.
How could this be useful for the owner of a server? Wouldn't it be better to pretend otherwise: open port -> closed?
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You can't pretend an open port is closed, because an open port is really just a service that's listening. You can't pretend-close it and still have that service work. The only thing you can do is firewalling off the entire service, but presumably, any competent distro will firewall off all services by default and any service listening publicly is doing so for a good reason.
I guess it comes down to whether they feel like it's worth obfuscating port scan data. If you deploy that across all of your network then you make things just a little bit more annoying for attackers. It's a tiny bit of obfuscation that doesn't really matter, but I guess plenty of security teams need every win they can get, as management is always demanding that you do more even after you've done everything that's actually useful.
indeed, a service on a port would no longer properly work. However, pretending that an open port is closed is possible the same way when pretending that's open
Maybe what you're referring to is along the lines of a port being open but the software on the other side of it not sending acknowledging responses?